This invention relates to a method of transmitting terminating call signals in a radiotelephone system and to a base station and a portable unit both of which are for use in the method. It is mentioned here that the portable unit may be a cordless telephone set, a handset, or the like, and conveyed by a vehicle, such as a land mobile, or a person and that the base station is fixed at a predetermined site and may therefore be called a fixed station.
As a radiotelephone system, a multiple channel access system is known wherein each of radio channels is used in common to a plurality of portable units. The multiple channel access system is very effective in view of significant utilization of the radio channels. In the multiple channel access system, a selected one of the radio channels is used to transfer a wide variety of control signals, such as a terminating call signal, between the base station and the portable units and may therefore be called a control channel. In this connection, the remaining channels may be referred to as speech channels. From this fact, it is readily understood that availability of the multiple channel access system greatly depends on a length of occupied time in the control channel.
On the other hand, the portable unit comprises a receiver portion driven by a battery. The receiver portion may not always be put into an active state because occurrence of the terminating call signal is intermittent or spontaneous. Under the circumstances, a battery saver circuit is included in the portable unit in order to save power consumption of the battery and to thereby extend a life time of the battery. More specifically, the battery saver circuit periodically puts the entirety of the receiver portion into transient active and inactive states during a first time interval and during a second time interval considerably longer than the first time interval, respectively. In other words, a battery saving operation is carried out in the battery saver circuit during the second time interval at a battery saving period. Preferably, the battery saving period is as long as possible in the light of saving of electric power of the battery.
In U.S. Pat. No. 4,419,765 issued to Wycoff et al, a receiver is disclosed wherein the battery saving operation must intermittently be interrupted after lapse of the first time interval to put the receiver portion into a receiving state when the terminating call signal is received during the first time interval. Thereafter, the intermittent interruption of the battery saving operation is changed to a continuous interruption when the terminating call signal is found to be destined to the portable unit in question after detection of the terminating call signal. Otherwise, the battery saving operation is restarted.
Let the terminating call signal be produced by the base station in the form of a succession of digital signals. In this event, the intermittent interruption of the battery saving operation becomes objectionably long because the detection of the terminating call signal should be made through a filter with reference to at least one of the digital signal succession, for example, during a couple of the digital signal successions. As a result, electric power of the battery might be wasted during the intermittent interruption.
It is possible to reduce occurrence of the intermittent interruption by extending the battery saving period. However, it takes a long time to access the portable unit in question. This means that the control channel is occupied by the base station for a long time on production of each terminating call signal.
To the contrary, the intermittent interruption frequently occurs in the portable unit and results in serious consumption of the battery, when the battery saving period is shortened.